There exist many circumstances in which it is necessary to accurately determine an individual's physical location. For example, an individual may require emergency assistance, but be unaware of his or her location. In this situation, the emergency services operator must be able to locate the individual in order to provide the proper aid. In another example, a third party (e.g., police, a family member, etc.) may need to quickly locate the individual.
One possible solution is to use the Global Positioning System (“GPS”). GPS may utilize satellite communication technology to estimate the position of a GPS receiver to within about 15 meters. This solution may also comprise the use of Assisted GPS (“A-GPS”) wherein the receiver may use terrestrial resources with better computation resources and a more direct line of sight to the GPS satellites to quicken the acquisition of a GPS location estimate. However, a typical wireless communication device (a.k.a., user equipment (“UE”)) does not contain the components necessary to use GPS. Furthermore, GPS does not work well if the UE lacks a relatively clear line of sight to a sizeable portion of the overhead sky.
A second solution for individuals carrying a UE may be to determine the cell in which the UE is located. Wireless networks are typically divided into geographic areas (i.e., cells) that are each serviced by a base station. When a UE is communicating via a wireless network (i.e., the UE is “connected” to that network), the UE will typically be assigned to one of these cells and transmit and receive signals via a corresponding base station. One can infer that a UE assigned to a specific cell will be located within (or within geographic proximity of) that cell. However, each cell can be quite large and so this limited information does not provide a very accurate estimate of an individual's location.
A farther refinement to the above estimate may be achieved by calculating the distance between a base station and the UE. As described above, when a UE is connected to a wireless network, it will typically transmit and receive signals via a designated base station 105, as shown in FIG. 2. The base station 105 may transmit a signal 201 to the UE 202. The signal 201 may be a signal designated for this purpose, or may be a signal carrying data from the wireless network to the UE 202. Because wireless transmissions travel at a finite speed (i.e., the speed of light), there is a first finite duration of time between the base station 105 transmitting signal 201 and the UE 202 receiving the signal. Because the speed of light is constant, this first duration may be used to calculate a distance 203. After the UE 202 receives the signal 201, it transmits a second signal 204 back to the base station 105. Similarly, there is a second finite duration of time between the UE 202 transmitting the signal 204 and the base station receiving the signal. This second duration may be used to calculate a distance 205. In some systems, it may be assumed that distances 203 and 205 will be roughly equal to each other. In this case, one may calculate the total duration between when the base station transmitted the signal 201 and when the base station received the signal 204 (i.e., the round trip time (“RTT”)), which includes the first and second signal propagation durations as well as any internal delays within the UE (e.g., processing delays). Dividing this time by half, accounting for any internal delays within the UE, and multiplying by the speed of light will provide a distance that is the average of the distances 203 and 205. In this way the RTT can be used to determine the approximate distance between the UE 202 and the base station 105. However, because this estimate is one-dimensional, this will generally only identify an arc 206 on which the UE 202 may be located. While this may be an improvement on merely identifying the cell in which the UE is located, it is still relatively imprecise.